In high voltage ac and dc power transmission lines it is necessary to deliver at ground potential an accurate value or replica of the amplitude and phase of the line current. This is then used for operating a variety of sophisticated relays that protect the line and make measurements of faults. It is also used for measuring the amount of ac power that the line is delivering to the customer (dc metering is not yet available). The foregoing applications require fast response and high accuracy.
Magnetic core current transformers have been used for the above tasks but at line voltage of 500 kv--ac and 250 kv-dc and above conventional ac current transformers and dc magnetic amplifiers are very heavy and expensive.
However, there have been a number of efforts to develop an electronic current transducer or transformer in which the required information is measured at line potential and transmitted to ground on a modulated beam of light or via radio waves. In a typical system a high speed electronic (analog or digital) converter at line potential converts the instantaneous current amplitude to a low level signal which is used to modulate a light-emitting diode. The modulated light is transmitted by a long fiber optic light guide to a ground unit where the modulated light is detected and converted back to an analog waveform duplicating the current waveform on the line. Some of the serious problems with such a system are that it is very difficult and expensive to combine the required wide dynamic range (e.g., 10,000), the fast sampling rate (2 msec. response time), and the accuracy (0.3% over a -40.degree. to 50.degree. C. temperature range).
Another technique uses Faraday Rotation where a beam of light, for example from a laser, is sent up through a hollow insulator from the ground, modified by the magnetic field of the current by polarization, and is returned to ground where this rotation is sensed. The foregoing techniques are illustrated in Hermstein U.S. Pat. No. 3,681,688 and Heintz U.S. Pat. No. 3,492,574. Hermstein improves accuracy as illustrated in FIG. 3 of his patent by the use of a second laser channel for temperature compensation. Heintz has a two channel fiber optic system: one channel carries pulse length information related to the magnitude of the line current and a second channel polarity.
Yet another problem in the prior art, especially with the advent of high voltage dc transmission grids, is that the dc current may flow in either direction in some of the branches. Then in addition to the above accuracy and response requirements a line current sensor must have the capability of measuring bipolar currents. This is especially difficult where light transmitting channels such as fiber optics are used since there can be no "negative light."